Range-from-focus techniques commonly are based on solving the standard lens law: ##EQU1## Typically, f (the focal length of the lens or lens system) is known. Therefore, by determining the value of v (the focus distance) corresponding to the distance between the lens center and an image plane which causes an object or image to be in focus, u (the depth value or range) can be calculated. Typically, to determine the v value, the distance (s) of the image plane from the lens is varied until the distance corresponding to the focus for an object or image is obtained. This focus determination is made by processing the detected image information according one or more well known focus determination algorithms. As used herein, s shall refer to the distance between the lens center and an image element of the imaging array. For a given scene point located at a given distance from the lens, the s value which produces the sharpest focus for the scene point is equivalent to the v value. That is to say, when a given scene point is in focus, s=v. In known imaging devices, the imaging elements along an image plane are typically aligned linearly and are each equidistant from a reference plane, e.g., a plane which passes through the lens center and which is perpendicular to the optical axis of the lens. For this arrangement, the s value (i.e., the distance from the lens center) of each imaging element is considered to be approximately equal. Therefore, for these arrangements, it is common to refer to the s value as being the distance of the image plane from the center of the lens.
Typically, in focus-based range determination methods, a depth estimate of a scene point is obtained by varying the focal-length (f) and/or the image plane distance (s). For simplicity, it will be assumed that the parameter being controlled is s. This means that the s value is changed by mechanically relocating the image plane, for example, by moving it towards or away from the lens to find the distance which causes the scene point to be in sharpest focus.
FIG. 1 depicts what is referred to as a sharp focus (SF) surface for a rectangular image plane imaged through a lens. The SF surface represents the set of scene points that will be imaged with sharp focus for some constant value of focal length (f) and focus distance (v). When an image is formed on an image plane which is perpendicular to the optical axis (assuming that the lens has no optical aberrations), the SF surface will be a surface that is approximately planar and normal to the optical axis (ignoring the depth of field effect). The size of the SF surface will be a scaled version of the size of the image plane, while its shape will be the same as that of the image plane. As the image plane distance from the lens, s, is changed, the SF surface moves away from or towards the camera. As a range of s values is traversed, the SF surface sweeps out a cone-shaped volume in three-dimensional space (the SF cone). The vertex angle of the SF cone represents the magnification or scaling achieved and is proportional to the f value. Since only those scene points within the SF cone can be imaged sharply, to increase the size of the imaged scene, the f value must be increased. Typically, however, the field of view (or frame) of an imaging apparatus is smaller than the entire visual field of interest, commonly referred to as a scene. Since in practice there is a limit on the usable range of f values, it is not possible to image a large scene in one viewing (or frame). Therefore, the camera must be panned to image different parts of the scene. "Panning" is the process of changing the parameters of an optical system to view a scene larger than that seen using some fixed parameter values. "Pan angle" is related to the parameters that are changed during "panning" and determines the part of the visual field being viewed by the optical system at any given time.
Consequently, typical range-from-focus techniques involve two mechanical steps. The first step involves sequentially panning over a range of pan angles to acquire images of the entire scene, a frame at a time. The second mechanical step involves, for each pan angle (or frame), finding the best focus distance or v value by mechanically relocating the image plane (for example, by moving it towards or away from the lens). The necessity for mechanically relocating the image plane to find the best v value for each pan angle makes this technique complex and time consuming, which is obviously undesirable.
Therefore, one drawback of the prior art is that it fails to provide an apparatus or method for determining a desired v value for each scene point, without conducting a dedicated mechanical search over a range of possible v values for each pan angle.